The ratio of chemically induced chromosome aberrations to gene mutations in barley a critical study

Abstract

A review of the literature on experiments with chemical mutagens in barley seeds shows great diversity as to the ratios of chromosomal aberrations to chlorophyll mutations, even for the same agent. These divergencies may frequently be attributed to an insufficient control of the specific conditions of aberration induction by alkylating agents. This study critically deals with the estimation of aberration frequencies suitable for exact comparisons. A method is described which allows relatively reliable estimations of aberration frequencies. The following aspects are considered: (1) the specific origin of aberrations, (2) the induced delay of cell divisions depending on the agents and their concentrations, and as a consequence of these two points, (3) the distribution of the aberration frequencies in dependence on the recovery times. First, it appears to be necessary to define the interphase stages of the meristem cell nuclei of the seed material. Most reliable results can be expected if the nuclei are arrested exclusively at the G 1 or G 2, respectively. Secondly, the aberrations should be scored after several recovery times separated by short intervals of equal length and covering at least the whole first and/or second mitotic cycle of germination. The maxima of aberrations evaluated in this way may be taken as a basis for exact comparisons. Several published data on ratios of aberrations to mutations obtained in experiments with ethyl methanesulphonate in barley are discussed critically. Obviously, the aberration frequencies in many reports were inferred under experimental conditions that were not fully adequate to the specific requirements. This may be the main reason for the great diversity in the ratios of chromosomal aberrations to chlorophyll mutations. Changes in the ratios of aberrations to mutations that are attributed to differences in treatment conditions, the metabolic state of the seeds, or the nature of the alkyl groups of alkylating agents may, in part, also be due to inadequate details of the experimental conditions. Thus reinvestigations will be necessary to discover their actual reasons.